1. Field of the Invention
The present invention relates to a polymeric fluorescent substance, and an organic electroluminescence device(hereinafter referred to as an "organic EL device") produced by using the polymeric fluorescent substance.
2. Description of the Related Art
An inorganic electroluminescence device using an inorganic fluorescent substance as a light emitting material (which may hereinafter referred to as an "inorganic EL device") has been used, for example, as a flat light source for back light or a display device such as a flat panel display and the like, however, high A.C. voltage has been required to emit light.
In view of the improvement of inorganic EL devices, a device having a double-layer structure comprising a light emitting layer of an organic fluorescent pigment and a layer of an organic charge transporting compound (Japanese Patent Kokai (Laid-Open) No. 59-194393) and a device comprising a polymer as a light emitting material (WO-A-9013148, Japanese Patent Kokai (Laid-Open) No. 3-244630) have been reported. These electroluminescence devices use organic materials having advantageous features so that low-voltage direct current drive, high luminance and multi-color light emission can be obtained easier than inorganic EL devices.
Polymeric light emitting materials which have hitherto been reported, as WO-A-9013148 discloses a poly-p-phenyene vinylene thin film prepared by forming a soluble precursor film on an electrode and subjecting the film to a heat treatment to convert it into a conjugated polymer. Japanese Patent Kokai No. 3-244630 discloses a conjugated polymer having a feature that is soluble in a solvent and does not require a heat treatment, such as poly-2,5-dialkoxy-p-phenylene vinylene and the like. Furthermore, Japanese Patent Kohyo -510483 discloses a poly-p-phenylene vinylene copolymer having a cholestanoxy group as a nuclear substituted side chain, which has good solubility.
Alternatively, as polymeric light emitting materials, a polymeric fluorescent substance containing a conjugated segment and a non-conjugated segment in a molecule and a poly-p-phenylene vinylene derivative wherein a cyano group is connected with a vinylene group are reported [Nature, Vol. 365, page 628 (1993)]. Examples of the polymeric fluorescent substance containing a conjugated part and a non-conjugated part in a molecule include a random copolymer of a 2,5-dimethoxy-p-phenylene ethylene structure and p-phenylene vinylene [Nature, Vol. 356, page 47 (1992)] and a polymer wherein a phenylene vinylene structure and an aliphatic hydrocarbon are connected with an ether linkage [Macromolecules, Vol. 26, page 1188 (1993)].
However, the poly(arylene vinylene) type polymeric fluorescent substance used in organic EL devices, which has hitherto been reported, must be subjected to a heat treatment when passing through a solvent-soluble precursor. Therefore, there was a limitation that only a substrate having a high heat resistance can be used. In order to obtain a solvent-soluble polymeric fluorescent substance, it is necessary to introduce a flexible repeating unit into a main chain or to introduce a flexible side chain having a high solvent affinity. As a result, there was a problem with a low heat resistance in the device because the polymeric fluorescent substance is susceptable to be softened at high temperature.
When an alkyl group or an alkoxy group is contained in a side chain, as the side chain length increses, the quantum yield of fluorescence becomes higher but the softening temperature becomes lower. Therefore, it is difficult to simultaneously satisfy a high efficiency and a high heat resistance. As described above, there is required a material which has a high quantum yield of fluorescence and a high light emitting efficiency of organic EL devices and is thermally stable.
Under these circumstances, the present inventors have intensively studied so as to further improve the light emitting efficiency, the heat resistance and the lifetime of the organic EL device by using a polymeric fluorescent substance in a light emitting layer. As a result, it has been found that a polymeric fluorescent substance having a specific structure at the side chain exhibits a high quantum yield of fluorescence and a high heat resistance, and an organic EL device can be easily produced using said polymeric fluorescent substance by coating it and, furthermore, this organic EL device exhibits a high light emitting efficiency, a high heat resistance and a long lifetime.